EP2208360B1

EP2208360B1 - Microphone arrangement comprising three pressure gradient transducers

Info

Publication number: EP2208360B1
Application number: EP07815180A
Authority: EP
Inventors: Friedrich Reining
Original assignee: AKG Acoustics GmbH
Current assignee: AKG Acoustics GmbH
Priority date: 2007-11-13
Filing date: 2007-11-13
Publication date: 2011-04-27
Anticipated expiration: 2027-11-13
Also published as: US20090190775A1; ATE507683T1; EP2208360A1; WO2009062212A1; CN101874411B; CN101874411A; DE602007014271D1

Abstract

A microphone arrangement includes pressure gradient transducers that include a diaphragm. Each pressure gradient transducer has a first sound inlet opening that leads to a front portion of the diaphragm and a second sound inlet opening that leads to a back portion of the diaphragm. The directional characteristic of each pressure gradient transducer includes an omni portion, a figure-eight portion, and a direction of maximum sensitivity in a main direction. The acoustic centers of the pressure gradient transducers lie within an imaginary sphere having a radius corresponding to about double the largest dimension of the diaphragm. Projections of the main directions of the pressure gradient transducers form angles between about 110° and about 130° in a base plane.

Description

The invention concerns a microphone arrangement consisting of three pressure gradient transducers, each with a diaphragm, with each pressure gradient transducer having a first sound inlet opening, which leads to the front of the diaphragm, and a second sound inlet opening, which leads to the back of the diaphragm, and in which the directional characteristic of each pressure gradient transducer comprises an omni portion and a figure-of-eight portion and has a direction of maximum sensitivity, the main direction, and in which the main directions of the pressure gradient transducers are inclined relative to each other. The invention also concerns a method for synthesizing one or more microphone signals from the microphone arrangement according to the invention.
A coincident arrangement of gradient transducers in the form of a so-called soundfield microphone (sometimes also found under the name B-format microphone) is disclosed in US Patent No. 4,042,779 A (and the corresponding DE 25 31 161 C1 . This is a microphone consisting of four pressure gradient capsules, with the individual capsules being arranged in tetrahedral form, so that the diaphragms of the individual capsules are essentially parallel to the imaginary surfaces of a tetrahedron. Each of these pressure gradient receivers delivers its own signal A, B, C or D and has a directional characteristic deviating from the sphere, which can be approximately represented in the form k + (1-k) × cos(θ), with θ denoting the azimuth, under which the capsule is exposed to sound, and with the ratio factor k indicating how large the percentage of omni signal is (in a sphere, k = 1, in a figure-of-eight, k = 0). The signals of the individual capsules are denoted A, B, C and D. The axis of the symmetry of directional characteristic of each individual microphone is perpendicular to the diaphragm and to the corresponding surface of the tetrahedron. The axes of symmetry of the directional characteristic of each individual capsule (also called the main direction of the individual capsule) therefore form an angle of about 109.5° with each other.
According to one calculation procedure, the four individual capsule signals are now converted to the so-called B-format (W, X, Y, Z): $W = ½ (A + B + C + D)$
$X = ½ (A + B - C - D)$
$Y = ½ (- A + B + C - D)$
$Z = ½ (- A + B - C + D)$
The forming signals are a sphere (W) and three figures-of-eight (X, Y, Z) that are orthogonal to each other. The latter are also arranged along the three spatial directions. In order to configure the frequency and phase response for all directions, so that a flat energy characteristic is achieved with respect to the frequencies in the audible range, it is essential to equalize the signals W, X, Y, Z. For the zero-order signal (W) and the first-order signals X, Y, Z, theoretical equalization characteristics are given in US 4,042,779 A , which depend on the frequency and effective distance of the center of the microphone capsules from the center of the tetrahedron.
The main directions of the figures-of-eight X, Y, Z are normal to the sides of a cube enclosing the tetrahedron. By linear combination of at least two of these B-format signals, an arbitrary (in the spatial direction and directional characteristic) microphone capsule can be synthesized. Deviations from the theory based on the use of real capsules and the failure to satisfy ideally the coincidence requirement cause a deterioration in the performance of the synthesized microphones.
Synthesizing or modeling of the microphone (as this is called in the technical jargon) occurs precisely in that the omni signal W is combined with one or more of the figure-of-eight signals X, Y, Z, taking into account a linear weighting factor "r". For directional characteristics in the area between a sphere and a cardioid, this can be derived for a synthesized capsule in the X-direction by means of the formula M = W + r × X, in which r can assume arbitrary values > 0. The level of the signal M so obtained must naturally be normalized, so that the desired frequency trend is obtained for the main direction of the synthesized capsule. If a synthesized capsule is now considered in any direction, additional weighting factors are necessarily used, since rotation of the synthesized capsule in any direction occurs by a linear combination of 3 orthogonal figures-of-eight (X, Y, Z).
The major advantage of the soundfield microphone is that it is possible, after storing the sound events recorded by the individual microphones, to alter the directional characteristic of the entire microphone by corresponding calculation of the individual signals, and therefore to adapt it in the desired manner even during playback or final production of a sound carrier. For example, it is therefore possible to focus on a corresponding soloist of an ensemble, to mask out unexpected and undesired sound events by influencing the directional characteristic, or to follow a moving sound source (for example, a performer on the stage), so that the recording quality always remains independent of the changed position of the sound source.
With the sound recordings of a soundfield microphone, the entire sound field (hence the name) can be described at any location in space over time, so that travel time differences, etc., are available for analysis during selected evaluation of the data.
The deviation from the ideal case, a point-like microphone, however, means that the coincidence conditions for small wavelengths are no longer satisfied and that distortions and artifacts will occur with respect to the frequency response and directional characteristic of the synthesized signal. A rotation of each individual gradient capsule of the soundfield microphone of 180°, so that each of the four diaphragm surfaces is brought closer to the center, has shown that artifacts cannot be eliminated at higher frequencies. Acoustic shadowing of the front microphone mouthpieces so arranged does not alter the limit frequency, up to which the calculation method applies.
There is a trade-off between the coincidence requirement and the attainable noise distance of the employed gradient capsules. The larger the individual diaphragm surface, the more noise distance can be achieved. However, this leads to a larger distance of the diaphragm surfaces to the center of the arrangement. An optimal solution now requires positioning of the 4 individual capsules as closely as possible to each other, so that the sound inlet on the back of the gradient transducer is influenced by the resulting structure of the closely positioned capsules. This means that the cavity formed in the interior of the microphone arrangement, and naturally also its delimitation by the microphone arrangement, as well as its mounts, etc., will act as an acoustic filter, which is added to the usual acoustic filtering by the sound paths that lead to the back of the individual capsules. The effect of this additional acoustic filter is frequency-dependent and has its strongest effect at frequencies at which the wavelength of the sound is essentially of the same order as the dimensions of the diaphragm or the dimensions of the entire soundfield microphone. In the soundfield microphones now employed, this strong effect lies essentially in the frequency range around 10 kHz, at which rejection, i.e., the frequency response from the direction from which the individual capsule is least sensitive becomes weakest and, in most cases, drops below 10 dB.
To compensate for these distortions, EP 1737 268 proposes to arrange a fixed element in the interior of the space: formed by the individual microphones, which fills up the free volume of this space by at least half. This expedient, however, is also insufficient for certain applications, so that there is a demand for a more efficient solution. Moreover, this expedient has no effect on non-ideal coincidence.
Another coincident microphone arrangement is disclosed by US 4,262,170 . Microphones arranged as close as possible next to each other with a directional characteristic according to the formula E = K + (1 - k) cos θ are oriented so that the directions of the maximum sensitivity point in different directions by an azimuthal angle. This type of arrangement is used to record Surround Sound, but has the drawback that, here again, the coincidence conditions cannot be optimally satisfied.
DE 44 98 516 C2 discloses a microphone array of three microphones arranged along a straight line, spaced more than 2.5 cm from each other. Coincidence is not present. Rotation of the directional characteristic, as in a soundfield microphone, is not possible, nor is it intended.
EP 1 643 798 A1 discloses a microphone that accommodates two boundary microphones in a housing. A boundary microphone is characterized by the fact that both the sound inlet opening that leads to the front of the diaphragm and the sound inlet opening that leads to the back of the diaphragm lie in the same surface of the capsule, the so-called boundary. By arranging both sound inlet openings a, b on one side of the capsule, a directional characteristic that is asymmetric to the axis of the diaphragm is achieved, for example, cardioid, hypercardioid, etc. Such capsules are described at length in EP 1 351 549 A2 and the corresponding US 6,885,751 A .
EP 1 643 798 A1 now describes an arrangement in which the capsules are arranged one above the other, either with sound inlet openings facing each other or facing away from each other. This system is used for noise suppression, but is not capable of appropriately emphasizing the useful sound direction, so that undesired interfering noise is also unacceptably contained in the overall signal. This microphone arrangement is fully unsuited for the recording of Surround Sound since shadowing effects from the housing enclosing all of the parts included in the arrangement of the capsules, one above the other, modify the sound field at the location of the sound inlet opening so strongly, that no conclusions can be drawing concerning the actual sound field prevailing in the room.
DE 10 195 223 T1 discloses a microphone arrangement consisting of transducer elements arranged in a circular manner, which are supposed to record a sound field in its entirety. 50 mm is stated as the ideal radius of this arrangement, which lies far from the coincidence condition, i.e. it does not satisfy the conditions necessary to achieve coincidence. The principle of recording is based on the fact that an attempt is made to draw conclusions concerning the sound field at other locations by measurements at specific points. In theory, this functions more or less well, but, in practice, the free field is so sensitively disturbed by the presence of objects (for example, spatial conditions in the immediate vicinity of the microphone, microphone mounts, etc.) that equalization functions that require a transformation of the signals in the desired frequency range, and therefore a considerable calculation capacity, cannot compensate for this.
Returning to the soundfield microphone, applications are generally restricted to the fact that only B-format signals W (sphere), X and Y (figures-of-eight) are used, since they seek to produce recording for common loudspeaker configurations, which are generally set up in one plane. In the soundfield microphone, two of the capsules are always situated with their main direction downward, which means that they react particularly sensitively to non-ideal microphone mounting or fastening under practical conditions. Such acoustic disturbances, based on the capsule arrangement, develop due to reflections on the mounting material, on the floor, etc. In addition, the capsules in the close arrangement are influenced in that the theoretically rotationally symmetric directional characteristic of the synthesized omni signal is disturbed.
In the soundfield microphone of the conventional type, the most widely used configuration (X-Y-plane) is achieved by the switching of four capsule signals. The B-format signals in the X-Y-plane are formed from microphone signals that meet at an angle of about 54° in all capsules under the influence of sound. If the directional diagram of a gradient transducer is considered, scattering of the rejection angle of the individual capsules has a stronger effect, the more the inlet direction deviates from the main direction (0°). Expressed otherwise, if two capsules with slightly different polar patterns exposed to sound from 0° differ only by the sensitivity so defined, at angles greater than 0°, the difference is increased by a percentage as a result of the different rejection angles.
There is now a demand for a microphone arrangement in which the signals of the individual transducers can be converted to B-format, but which do not have the drawbacks known from the prior art. In the first place, coincidence is to be guaranteed and optionally improved. Shadowing effects, which arise because the individual capsules shade each other, are to be strongly reduced or not occur at all. Acoustic disturbance from spatial conditions in the immediate vicinity of the microphone arrangement and the dependence on capsule tolerances (for example, deviations in the manufacturing process) are to be minimized. The versatile possibilities of use of a soundfield microphone should not be restricted.
These objectives are achieved with a microphone arrangement of the type just mentioned, in that the acoustic centers of the three pressure gradient transducers lie within an imaginary sphere whose radius corresponds to the double of the largest dimension of the diaphragm of a pressure gradient transducer, so that the projections of the main directions of the three pressure gradient transducers into a base plane spanned by the first sound inlet openings of the three pressure gradient transducers enclose an angle with each other, whose values lie between 110° and 130°.
The first criterion ensures the necessary coincident position of all transducers. In a more preferable embodiment the acoustic centers of the pressure gradient transducers lie within an imaginary sphere whose radius corresponds to the largest dimension of the diaphragm of a transducer. Increasing the coincidence by moving the sound inlet openings together exceptional results may be achieved.
The first feature mentioned above determines the coincidence of the microphone arrangement, and the orientation of the main directions permits the synthesis of a B-format.
The patient according to the invention is characterized by the fact that, starting from the signals of the three pressure gradient capsules, a B-format is formed, which contains an omni signal and two figure-of-eight signals orthogonal to each other.
"Synthesized directional characteristic" is understood to mean an arbitrary combination of individual B-format signals, for example, a sphere (W) with at least one additional B-format signal (a figure-of-eight), and also their further processing, such as equalization, bundling, rotation, etc. The individual signals are then considered with a corresponding weighting.
The expression "directional characteristic" is understood to mean not only the directional characteristic of the real capsules, but of signals in general. These signals can be composed of other signals (for example, B-format signals) and have complicated directional characteristics. Even if such directional characteristics cannot be achieved under some circumstances with individual real capsules, the expression "directional characteristic" is used in this context, since in this way it is clearly established from which spatial areas the formed or synthesized signals preferably yield acoustic information.
The invention is further explained below by means of drawings. Figure 1 shows a microphone arrangement according to the invention, consisting of three gradient transducers,

Figure 1A shows a variant of a microphone arrangement according to the invention,
Figure 1B shows another variant with the pressure gradient capsules within a common housing,
Figure 1C and 1D show the arrangement at a boundary,
Figure 2 shows another variant of the invention,
Figure 2A shows a variant in which the gradient capsules from Figure 2 are embedded within a boundary,
Figure 3 shows a gradient transducer with sound inlet openings on opposite sides of the capsule housing,
Figure 4 shows a gradient transducer with sound inlet openings on the same side of the capsule housing,
Figure 5 shows the directional characteristic of the individual gradient transducers from the z-direction,
Figure 6 shows the directional characteristics from the y-direction,
Figure 7 shows a sectional view of the microphone arrangement, consisting of four gradient transducers along line I-I of Figure 7A,
Figure 7A shows the top view of the microphone arrangement in Figure 7,
Figure 8 shows the directional characteristics of the gradient transducer according to
Figure 7 from the y-direction,
Figure 9 shows a block diagram to produce the B-format signals,
Figure 10 shows a block diagram of the expanded signal processing unit,
Figure 10A shows the depiction from Figure 10, with the directional characteristics inserted,
Figure 11 shows a block diagram of the spectral subtraction unit in detail,
Figure 12 shows a simplified circuit in contrast to Figure 10.
Figure 13 shows schematically the inventive concept of coincidence.
Figure 1 shows a microphone arrangement 10, according to the invention, constructed from three pressure gradient transducers 1, 2, 3. The directional characteristic of each transducer consists of an omni portion and a figure-of-eight portion. The directional characteristic can essentially be represented as P(θ) = k + (1 - k) × cos(θ), in which k denotes the angle-independent omni portion and (1 - k) × cos(θ) denotes the angle-dependent figure-of-eight portion. An alternative mathematical description of the directional characteristic is treated further below. As follows from the direction distribution of the individual transducers sketched in the lower portion of Figure 1, the present case involves a gradient transducer with a cardioid characteristic. In principle, however, all gradients that are derived from the combination of an omni and figure-of eight are conceivable, for example, hypercardioids.

The gradient transducers 1, 2, 3 in the depicted practical example lie in an x-y-plane, in which their main directions 1c, 2c, 3c - the directions of their maximum sensitivity - are inclined relative to each other by the azimuth angle ϕ. The angle ϕ, between two main directions, can take on values between 110° and 130°, but preferably about 120°. In principle, any type of gradient transducer is suitable for implementation of the invention, but the depicted variant is particularly preferred because this is then a flat transducer or so-called boundary microphone, in which the two sound inlet openings lie on the same side surface, i.e., boundary.
Figure 3 and Figure 4 show the difference between a "normal" gradient capsule and a "flat" gradient capsule. In the former, as shown in Figure 3, a sound inlet opening "a" is situated on the front of the capsule housing 4 and a second sound inlet opening "b" is situated on the opposite back side of the capsule housing 4. The front sound inlet opening "a" is connected to the front of diaphragm 5, which is tightened on a diaphragm ring 6, and the back sound inlet opening "b" is connected to the back of diaphragm 5. The arrows show the path of the soundwaves to the front or back of the diaphragm 5. In the area behind electrode 7, there is an acoustic friction means 8 is present in most cases, which can be designed in the form of a constriction, a non-woven or a foam.
In the flat gradient capsule in Figure 4, also called a boundary microphone, both sound inlet openings a, b are provided on the front of the capsule housing 4, in which one leads to the front of diaphragm 5 and the other leads to the back of diaphragm 5 via a sound channel 9. The advantage of this type of transducer is that it can be incorporated in a boundary 11, for example, a console in a vehicle; also, due to the fact that acoustic friction means 8, for example, a non-woven, foam, constrictions, perforated plates, etc., can be arranged in the area next to diaphragm 5, a very flat design is made possible.
By the arrangement of both sound inlet openings a, b on one side of the capsule, a directional characteristic asymmetric to the diaphragm axis is achieved, for example, cardioid, hypercardioid, etc. Such capsules are described at length in EP 1 351 549 A2 and the corresponding US 6,885,751 A .
It applies, for all capsules, that the front of the diaphragm is the side that can be reached relatively unhampered by sound, whereas the back of the diaphragm can only be reached after passing through an acoustically phase-rotating element by the sound. The sound path to the front is generally shorter than the sound path to the rear.
Returning to the microphone arrangement according to the invention as shown in Figure 1, there is now the special feature that the three gradient capsules 1, 2, 3 are oriented toward each other, so that the sound inlet openings 1a, 2a, 3a leading to the front of the corresponding diaphragm lie as close as possible to each other, whereas the sound inlet openings 1b, 2b, 3b leading to the back of the diaphragm lie on the periphery of the arrangement. In the subsequent explanation, the point of intersection of the lengthened connection lines that join the front sound inlet opening 1a, 2a and 3a to the rear sound inlet opening 1b, 2b and 3b is viewed as the center of the microphone arrangement. The front sound inlet openings 1a, 2a, 3a of the three transducers 1, 2, 3, also called mouth pieces, are therefore situated in the center area of the arrangement. The coincidence of the two transducers can be strongly influenced by this expedient.
The coincidence comes about in that the acoustic centers of the gradient transducers 1, 2, 3 lie as close as possible to each other, preferably at the same point. The acoustic center of a reciprocal transducer is defined as the point from which omni waves seem to be diverging when the transducer is acting as a sound source. The paper "A note on the concept of acoustic center", by Jacobsen, Finn; Barrera Figueroa, Salvador, Rasmussen, Knud; Acoustical Society of America Journal, Volume 115, ) examines various ways of determining the acoustic center of a source, including methods based on deviations from the inverse distance law and methods based on the phase response. The considerations are illustrated by experimental results for condenser microphones.
The acoustic center can be determined by measuring spherical wave fronts during sinusoidal excitation of the acoustic transducer at a certain frequency in a certain direction and at a certain distance from the transducer in a small spatial area--the observation point. Starting from the information concerning the spherical wave fronts, a conclusion can be drawn concerning the center of the omni wave--the acoustic center.
An also detailed presentation of the concept of acoustic center, applied to microphones, is found in "The acoustic center of laboratory standard microphones" by Salvador Barrera-Figueroa and Knud Rasmussen; The Journal of the Acoustical Society of America, Volume 120, ). What was described in this paper is presented below as one of the many possibilities for determining the acoustic center:
For a reciprocal transducer, such as the condenser microphone, it makes no difference whether the transducer is operated as a sound emitter or sound receiver. In the above paper, the acoustic center is defined by the inverse distance law: $p (r) = j \frac{ρ * f}{2 * r_{t}} M_{f} * i * e^{- r^{* r_{t}}}$

r₁ Acoustic

ρ Density of air

f Frequency

M_f Microphone sensitivity

i Current center

γ Complex wave propagation coefficient
The results pertain exclusively to pressure receivers. The results show that the center, which is defined for average frequencies (in the range of 1 kHz), deviates from the center defined for high frequencies. In this case, the acoustic center is defined as a small area. For determination of the acoustic center of gradient transducers, an entirely different approach is used here, since formula (1) does not consider the near-field-specific dependences. The question concerning the acoustic center can also be posed as follows: Around which point must a transducer be rotated, in order to observe the same phase of the wave front at the observation point.
In a gradient transducer, one can start from a rotational symmetry, so that the acoustic center can be situated only on a line normal to the diaphragm plane. The exact point on the line can be determined by two measurements - most favorably from the main direction, 0°, and from 180°. In addition to the phase responses of these two measurements, which determine a frequency-dependent acoustic center, for an average estimate of the acoustic center it is simplest to alter in the time regime the rotation point around which the transducer is rotated between measurements, so that the impulse responses are maximally congruent (or, stated otherwise, so that the maximum correlation lies in the center between the two impulse responses).
The described capsules, in which the two sound inlet openings are situated on a boundary, now possess the property that their acoustic center is not the diaphragm center. The acoustic center lies closest to the sound inlet opening that leads to the front of the diaphragm, which therefore forms the shortest connection between the boundary and the diaphragm. The acoustic center could also lie outside of the capsule.
The inventive coincidence criterion requires, that the acoustic centers 101, 201, 301 of the pressure gradient capsules 1, 2, 3 lie within an imaginary sphere O, whose radius R is double of the largest dimension D of the diaphragm of a transducer.
In a more preferable embodiment the acoustic centers of the pressure gradient transducers lie within an imaginary sphere whose radius corresponds to the largest dimension of the diaphragm of a transducer. By increasing the coincidence by moving the sound inlet openings together exceptional results may be achieved.
The preferred coincidence condition, which is also shown schematically in Figure 13, has proven to be particularly preferred for the transducer arrangement according to the invention: In order to guarantee this coincidence condition, the acoustic centers 101, 201, 301 of the pressure gradient capsules 1, 2, 3 lie within an imaginary sphere O, whose radius R is equal to the largest dimension D of the diaphragm of a transducer. The size and position of the diaphragms 100, 200, 300 are indicated in Fig. 13 by dashed lines.
As an alternative, this coincidence condition can also be defined in that the first sound inlet openings 1a, 2a, 3a lie within an imaginary sphere whose radius is smaller than the largest dimension of the diaphragm in the pressure gradient transducer. Use of the largest diaphragm dimension (for example, the diameter in a round diaphragm, or a side length in a triangular or rectangular diaphragm) to determine the coincidence condition is accompanied by the fact that the size of the diaphragm determines the noise distance and therefore represents a direct criterion for designing the acoustic geometry. It is naturally conceivable that the diaphragms 100, 200, 300 do not have the same dimensions. In this case, the largest diaphragm is used to determine the preferred criterion.
In the depicted practical example, the three gradient transducers 1, 2, 3 are arranged in a plane. The connection lines of the individual transducers, which connect the front and rear sound inlet openings to each other, are inclined with respect to each other by an angle of about 120°. The front sound inlet openings lie on the corners of a preferably equilateral triangle. The rear sound inlet openings also lie on the corners of an external, preferably equilateral triangle. This arrangement of three gradient transducers is based on the requirement for the best possible coincidence.
Figure 1A shows another variant of the invention, in which the gradient capsules are not arranged in a plane, but on an imaginary omni surface. This can be the case under practical conditions, when the sound inlet openings of the microphone arrangement are arranged on a curved boundary, for example, the console of a vehicle.
The curvature makes it so that, on the one hand, the distance to the center is reduced (which is desirable, because the acoustic centers lie closer together), and that, on the other hand, the mouthpiece openings are therefore somewhat shaded. In addition, this alters the directional characteristic of the individual capsules to the extent that the figure-of-eight portion of the signal becomes smaller (from a hypercardioid, then a cardioid). In order to not let the adverse effect of shadowing get out of control, the curvature should preferably not exceed 60°. In other words: the capsules are placed on the outer surface of an imaginary cone whose surface line encloses an angle of at least 30° with the cone axis.
The sound inlet openings 1a, 2a, 3a that lead to the front of the diaphragm lie in a plane, hereafter referred to as base plane, whereas the sound inlet openings 1b, 2b, 3b lie outside of this plane. The main directions of the individual gradient transducers 1, 2, 3 again each enclose an angle of 120° with each other. This orientation of the main directions represents a preferred variant. Deviations from the 120° arrangement by ± 10°, however, are also possible and lie within the scope of the present invention. The orientation of the main directions of the three gradient transducers at a 120° spacing permits the formation of the B-format, which is further discussed below.
As in the practical example with capsules arranged in the plane, in this practical example, the main directions of the transducers are inclined relative to each other by an azimuth angle ϕ, i.e., they are not only inclined relative to each other in a plane of the cone axis, but the projections of the main directions are also inclined relative to each other in a plane normal to the cone axis.
In the arrangement of Figure 1A, the acoustic centers of the three gradient transducers also lie within an imaginary sphere whose radius is equal to the double of the largest dimension of the diaphragm of the transducer. By this spatial proximity of the acoustic centers, the coincidence required for the invention, especially for the formation of the B-format, is achieved. As in the variants of Figure 1, the capsules depicted in Figure 1A are also preferably arranged on a boundary, for example, embedded in it.
Generally, in arrangements of several acoustic transducers, there are shadowing effects that can restrict the range of use (for example, the employable frequency range). If the transducers are used in or on a boundary for implementation of the microphone arrangement of the type mentioned in the introduction, all the shadowing effects are eliminated.
Possibilities of arranging capsules on a boundary are shown in Figure 1C and 1D. In Figure 1C, the capsules sit on the boundary 20, whereas they are embedded in boundary 20 in Figure 1D and are flush with their fronts with said boundary.
Figure 1B shows another variant in which the pressure gradient capsules 1, 2, 3 are arranged within a common housing 21, in which the diaphragms, electrodes and mounts of the individual transducers are separated from each other by intermediate walls. The first sound inlet openings 1a, 2a; 3a that lead to the front of the diaphragm and the second sound inlet openings 1b, 2b, 3b that lead to the back of the diaphragm can no longer be seen from the outside. The surface of the common housing 21, in which the sound inlet openings are arranged, (referring to an arrangement according to Figure 1) can be a plane or (referring to an arrangement according to Figure 1A) a curved surface. The boundary itself can be designed as a plate, console, wall, cladding, etc.
Figure 2 shows another variant of the invention that is constructed without a one-sided sound inlet microphone. In each of the pressure gradient transducers 1, 2, 3, the first sound inlet openings 1a, 2a, 3 a are arranged on the front of the capsule housing and the second sound inlet openings 1b, 2b, 3b are arranged on the back of the capsule housing. The first sound inlet openings that lead to the front of the diaphragm then face each other and again satisfy the requirement that they lie within an imaginary sphere whose radius is equal to the double of the largest dimension of the diaphragm of a pressure gradient transducer. The main directions (shown as arrows in Figure 2) of the three gradient transducers point in a common center area of the microphone arrangement according to the invention. The projections of the main directions again enclose an angle of 120° with each other in a plane in which the first sound inlet openings 1a, 2a, 3a or their centers lie, already defined above as the base plane. Deviations of ± 10° lie within the scope of the invention.
Figure 2A shows a variant in which the gradient capsules from Figure 2 are embedded within a boundary 20. It must then be kept in mind that the sound inlet openings are not covered by the boundary 20.
The signal processing of the individual capsule signals to a synthesized total signal will be described further below. The particularity consists in the fact that the partial signals W, X, Y applied in the most often used B-format can be formed from only three capsule signals. A set of signals, consisting of an omni signal and at least two figure-of-eight signals, can now be viewed in generalized manner as the B-format. In its general form, the B-fonnat consists of an omni signal and at least two figure-of-eight signals. In the present case, the B-format contains an omni signal and two figure-of-eight signals. These partial signals are therefore also referred to as a flat B-format.
Figure 9 shows how a flat B-format is formed from the individual capsule signals K1, K2 and K3 (the area separated by the dashed line with the capsule signal K4 is optional and will be described later). The B-format contains an omni signal W, an X-component of the B-format, and a Y-component of the B-format. The corresponding basic calculation procedure, which still requires no normalization, reads: $W = K 1 + K 2 + K 3$
$X = K 2 - K 3$
$Y = (2 \times K 1) - K 2 - K 3$

and is implemented by the circuit according to Figure 9. W is the omni signal and X and Y are the orthogonal figure-of-eight signals.
If normalization is now carried out, the B-format signals assume the form described below: The characteristics of each individual gradient capsule can also be described by the formula: $Kx = \frac{1}{a + b} (a + b \cos (θ))$

in which a represents the weighting factor of the omni portion and b represents the weighting factor for the gradient portion. For values a = 1, b =1, we obtain a cardioid; for values a = 1 and b = 3, we obtain a hypercardioid.
Three of the four B-format signals are calculated with consideration of the directional characteristic normalized to 1: $W = \frac{(K 1 + K 2 + K 3)}{3} * \frac{(a + b)}{a}$
$X = \frac{(K 2 - K 3)}{\sqrt{3}} * \frac{(a + b)}{b}$
$Y = \frac{(2 K 1 - K 2 - K 3)}{3} * \frac{(a + b)}{b}$
It should be noted here that the directional characteristic of the employed gradient capsules is included in these formula, which is not the case in US Patent 4,042,779 A , but is also valid there. In the mentioned patent, cardioids are used, but there is no indication that the transformation procedures, and especially the ratio between the B-format signals of the zero order (omni) and first order (figure-of-eight) depend on the directional characteristic of the employed capsules.
W represents the omni signal, which is an essentially omnidirectional signal. X and Y each represent a figure-of-eight lobe, whose axis of symmetry is parallel to the plane of the microphone. X and Y are orthogonal to each other and are therefore inclined by 90° relative to each other. By the combination of omni signal W with at least one of the figure-of-eight signals X, Y, any arbitrary directional characteristic can now be generated. By the linear combination of X and Y with corresponding weighting factors, the figure-of-eight can be rotated within the x-y-plane. By the linear combination of this rotated figure-of eight with the omni signal, the main direction of the synthesized signal can be rotated in different directions.
This linear combination can generally be written as the synthesized signal $M (q, r, s) = q \times W + r \times X + s \times Y,$

in which q, r, s represent the weighting factors, with which the B-format signals are incorporated in the final signal M.
In a particularly preferred variant shown in Figure 7, the microphone arrangement according to the invention includes an additional gradient transducer 4. The additional gradient transducer 4 has a directional characteristic containing at least one figure-of-eight portion.
Figure 7 shows a particularly preferred variant of the invention that has an additional pressure gradient transducer 4 in addition to the three pressure gradient transducers 1, 2, 3 (also called base pressure-gradient transducers for distinction), which is arranged centrally under the three base pressure- gradient transducers 1, 2, 3. The additional transducer 4 in the present case is a transducer shown in to Figure 3. 4a denotes the first sound inlet opening, which leads to the front of the diaphragm, whereas 4b represents the second sound inlet opening to the back of the diaphragm. Both sound inlet openings 4a, 4b lie in the axis of symmetry of the microphone arrangement. Figure 7A shows the arrangement in the section along I-I. This additional fourth transducer can have a pure figure-of-eight characteristic K4 = cos(θ), or can additionally contain an omni portion k. In the latter case, the signal of this gradient transducer can therefore be represented in the form: $K 4 = k + (1 - k) \times \cos (θ) or again as K 4 = \frac{1}{a + b} (a + b \cos (θ)) .$
The figure-of-eight portion is then oriented so that the axis of symmetry 4c of the figure-of-eight is normal to the base plane already defined above (i.e., the plane that is spanned by the first three sound inlet openings of the pressure gradient transducers 1, 2, 3). Even if the additional pressure gradient capsule 4 contains an omni portion, it applies that the main direction 4c of the fourth capsule is essentially normal to the base plane. By means of this fourth capsule, a complete B-format can be formed, consisting of a sphere W and three orthogonal figures-of-eight X, Y, Z.
An additional pressure gradient capsule 4 can also supplement the microphone arrangements shown in Figure 1, Figure 1A, and Figure 1B as well as Figure 2, Figure 2A. Orientation of the individual directional characteristics can be deduced from Figure 5, 6 and 8, in which Figure 5 and 6 show the case with three capsules and Figure 8 shows the case with the additional capsule 4. During the use of the additional pressure gradient capsule, however, it must be kept in mind that the rear sound inlet must not be covered, i.e., that the entire capsule arrangements must not be arranged in a boundary.
Figure 9 shows a block diagram to produce the B-format signals. The signals of the individual transducers are digitized by A/D transducers and the frequency responses of the individual transducers are equalized with respect to each other by means of filters F1, F2 and F3. By corresponding summations and subtractions, as well as multiplications, the signals are modified according to the equations for the B-format and weighted or normalized by means of amplifiers 71, 72, 73 and 74, so that normalized B-format signals X, W, Y, Z are formed according to the above formulas.
The optionally contained omni portion k of signal K4 can be compensated by the omni signal W already obtained from the three capsules 1, 2, 3 by filtering the microphone signal K4 by means of linear filter unit F3, so that after the filter, during sound exposure from any direction in the x-y-plane, the same signal as the W signal is formed after filter F3. Expressed otherwise, the omni signal is derived by measuring the impulse response from a direction across the main direction of the gradient transducer K4.
Because of this, during subtraction Z = K4 (filtered with F3) - W, a pure gradient signal with a figure-of-eight characteristic is left, whose axis of symmetry is normal to that of signals X and Y. This gradient signal is therefore in the z-direction, which is normal to the boundary (if this is a plane). The gradient signal Z can be adjusted by linear filtering in its properties to the frequency response and sensitivity of the X- and Y-signals. Because of this, as in an ordinary soundfield microphone, four B-format signals that can be arbitrarily combined with each other are obtained. The general expression therefore reads: $M (q, r, s, t) = q \times W + r \times X + s \times Y + t \times Z .$
The areas of application of such a soundfield microphone are numerous and extend to use in a vehicle, aircraft, for recording of music, conferences, etc..
A particularly interesting aspect of the invention is to be worked out below. Synthesized microphone signals M1, M2, and optionally M3, serve as the basis, which were calculated according to the formula: $M (q, r, s) = q \times W + r \times X + s \times Y$

or $M (q, r, s, t) = q \times W + r \times X + s \times Y + t \times Z .$
The synthesized signals M1, M2 and M3 now have directional characteristics that are oriented as in the figure. These are cardioids, whose main directions lie in one plane and are inclined with respect to each other by about 120°. The following example will describe the synthesized signals M1, M2 and M3 by means of this orientation, but in principle it is not restricted to this. Any arbitrary combination of signals would be conceivable.
Figure 10 shows a schematic block diagram between outputs, at which the synthesized signals M1 and M2 lie, and shows the output 31 of the signal processing unit 30. The synthesized signals, if they were not digitized anyway already, are initially digitized with A/D transducers (not shown). Subsequently, the frequency responses of all synthesized signals are compared to each other, in order to compensate for manufacturing tolerances of the individual capsules. This occurs by linear filters 32, 33, which adjust the frequency responses of the synthesized signals M2 and M3 to those of synthesized signal M1. The filter coefficients of linear filters 32, 33 are determined from the impulse responses of all participating gradient transducers, with the impulse responses being measured from an angle of 0°, the main direction. An impulse response is the output signal of a transducer, when it is exposed to a narrowly limited acoustic pulse in time. In determining the filter coefficients, the impulse responses of transducers 2 and 3 are compared with that of transducer 1. The result of linear filtering according to Figure 5 is that the impulse responses of all gradient transducers 1, 2, 3, after passing through, the filter, have the same frequency response. This expedient serves to compensate for deviations in the properties of the individual capsules.
Subsequently in the block diagram, a sum signal f1 + f2 and a difference signal f1 - f2 are formed from the filter signals f1 and f2 that result from M1 and M2 by filtering. The sum signal is dependent on the directional characteristic and its orientation in space, and therefore dependent on the angle of the main directions of the individual signals M1, M2 relative to each other, and contains a more or less large omni portion.
At least one of the two signals f1 + f2 or f2 - f1 is now processed in another linear filter 34. This filtering serves to adjust these two signals to each other, so that the subtraction signal f2 - f1 and the sum signal f1 + f2, which has an omni portion, have the maximum possible agreement when overlapped.
In the present case, the subtraction signal f2 - f1, which has a "figure-of-eight" directional characteristic, is inflated or contracted in filter 34 as a function of the frequency, so that maximum rejection in the resulting signal occurs when it is subtracted from the sum signal. The adjustment in filter 34 occurs for each frequency and each frequency range separately.
Determination of the filter coefficients of filters 34 also occurs via the impulse responses of the individual transducers. Filtering of the subtraction signal f2 - f1 gives the signal s2; the (optionally filtered) summation signal f1 + f2 - in the practical example with only two synthesized signals M1, M2 - gives the signal s1 (the portion of the signal, processing unit 30, shown on the right side of the dashed separation line, is not present during use of only two signals M1, M2).
However, three synthesized signals M1, M2, M3 can be evolved in signal processing (to the right of the separation line in Figure 10). The signal f3, made equal to the frequency response of signal M1 in linear filter 33, is now multiplied by amplification factor v and subtracted as v × f3 from the sum signal f1 + f2. The resulting signal s1 now corresponds to (fl + f2) - (v × f3), in the case of three signals.
It is initially established by the amplification factor v, in which direction the useful direction should lie, i.e., that spatial direction that is to be strongly limited by the directional characteristic of the synthesized total signal. The possible useful directions are unrestricted, in principle, because the synthesis signals M1, M2 and M3 can be arbitrarily rotated. For example, if factor v is very small, the effect of the third synthesis signal M3 on the total signal is limited and the sum signal f1 + f2 dominates relative to signal v × f3. If, on the other hand, the amplification factor v is negative and large, the individual signal v × f3 dominates over the sum signal f1 + f2 and the useful sound direction, or the direction in which the synthesized total signal directs its sensitivity, is therefore rotated by 180° with reference to the former case. By variation of factor v, this expedient permits a change in sum signals, so that an arbitrary directional characteristic is generated in the desired direction.
This bundling mechanism can be applied to all signal combinations. For the direction to which bundling is supposed to occur, an intrinsic spectral subtraction block is required. The signal processing steps occurring before the spectral subtraction block can be combined to the extent that only factor v need be different for two opposite directions, whereas all other preceding steps and branches remain the same for these two directions.
The spectral subtraction applied to the two intermediate signals s1 and s2 and occurring in block 40 is further explained below. Figure 11 shows the individual components of a spectral subtraction block 40 in detail and pertains to calculation at the digital level. It should be briefly mentioned here that the A/D conversion of the signals also can only occur before spectral subtraction block 40 and the filtering and signal combinations conducted before this occurs on the analog plane.
Two signals s1 (n) and s2(n) serve as the input of block 40 in the time range derived from the signals that were recorded at the same time and at the same point (or at least in the immediate vicinity). This guarantees the coincident arrangement of transducers 1, 2, 3; s1 (n) represents the signal that has the most useful signal portions, whereas s2(n) represents the signal that contains more interference signals, in which signal s2(n) is characterized by the fact that it has a zero-position, in the viewing of the polar diagram; in the useful sound direction; n represents the sample index and s(n) therefore corresponds to a signal in the desired time range.
The unit marked 50 generates individual blocks with a block length N = L + (M - 1) from the continuously arriving samples. L represents the number of new data samples in the corresponding block, whereas the remainder (M - 1) of samples was also already found in the preceding block. This method is known in the literature as the "overlap and save" method and is described in the book "Digital Signal Processing" by John G. Proakis and Dimitris G. Manolakis (Prentice Hall), among others, on page 432.
The N samples contained in a block are then conveyed to the unit designated 51 at the times at which M - 1 samples have reached unit 50 since the preceding block. Unit 51 is characterized by the fact that, in this area, processing occurs in a block-oriented manner. Whereas the signal s1 (n, N) packed into blocks reaches unit 51, the unit 52 is provided for the signal s2 (n, N) packed into blocks in the same way.
In units 51, 52, the end samples of signals s1 and s2 combined into a block, are transformed by FFT (fast Fourier transformation), for example, DFT (discrete Fourier transformation), into the frequency range. The signals S1 (ω) and S2(ω) that form are broken down in value and phase, so that the value signals | S1 (ω) | and | S2 (ω) | occur at the output of units 51 and 52. By spectral subtraction, the two value signals are now subtracted from each other and produce ( | S1(ω) | - | S2(ω) | ).
Subsequently, it applies that the resulting signal ( | S1(ω) | - | S2(ω) | ) is transformed back to the time domain. For this purpose, the phase Θ1(ω), which was separated in unit 51 from signal S1 (ω) = | S1(ω) | × Θ1 (ω) and which, like the value signal | S1(ω) |, also has a length of N samples, is used during the back-transformation. The back-transformation occurs in the one unit 53 by means of IFFT (inverse fast Fourier transformation), for example, IDFT (inverse discrete Fourier transformation) and is carried out based on the phase signal Θ1(ω) of S1(ω). The output signal of unit 53 can therefore be represented as IFFT. [( | S1(ω) | - | (S2(ω) | ) × exp(Θ1(ω)].
The so-generated N samples of long digital time signal S12(n, N) is fed back to processing unit 50, where it is incorporated in the output data stream S12(n) according to the calculation procedure of the "overlap and save" method.
The parameters that are necessarily obtained in this method are block length N and rate (M - 1)/fs [s] (with sampling frequency fs), with which the calculation or generation of a new block is initiated. In principle, in any individual sample, an entire calculation could be carried out, provided that the calculation unit is fast enough to carry out the entire calculation between two samples. Under practical conditions, about 50 ms has proven useful as the value for the block length and about 200 Hz as the repetition rate, in which the generation of a new block is initiated.
The described method of spectral subtraction merely represents one possibility among many. Spectral subtraction methods per se represent methods known in the prior art.
The signal processing just described (Figure 10 and 10A), in which a signal narrowly bundled in a specific direction can be produced, starting from B-format signals, can also be implemented more simply and directly. Figure 12 shows a corresponding circuit for three B-format signals W, X, Y to the synthesized signals s1 and s2. The subsequent spectral subtraction block 40 remains the same. The amplifiers 61 to 65 weigh the individual B-format signals according to the direction in which one intends to direct a narrow lobe of the directional characteristic. The filter 34 ensures that during the spectral subtraction of signal s1 from s2, the resulting signal s12 has minimal energy. It is again a situation in which the phase of signal s1, which also contains omni portion (W), is used in order to provide the subtracted signal with this phase. As already described at length above, this expedient serves to show the original character of the useful signal. A common feature of Figure 10 and 10A and Figure 12 is that an attempt is made to generate a signal s1 that has an omni portion W, in addition to figure-of-eight portions X and Y, and the purest possible figure-of-eight signal s2.
An essential advantage of the method according to the invention is obtained by the fact that the synthesized output signals s12(n) contain phase information from the special directions that point to the useful sound source, or are bundled on it; s1, whose phase is used, is the signal that has increasing useful signal portions, in contrast to s2. Because of this, the useful signal is not distorted and therefore retains its original sound.
The functional method and effect of the invention are particularly made apparent by means of the directional effect of the individual intermediate signals. Figure 10A shows the synthesized directional characteristics of the individual combined signals M1, M2, M3 and the intermediate signals, in which the amplitudes are in each case normalized to the useful sound direction designated with 0°, i.e., all the polar curves and those during sound exposure from a 0° direction are normalized to 0 dB. The output signal 31 then has a directional characteristic bundled particularly strongly in one direction.

Claims

Microphone arrangement comprising three pressure gradient transducers (1, 2, 3), each with a diaphragm, with each pressure gradient transducer (1, 2, 3) having a first sound inlet opening (1a, 2a, 3a), which leads to the front of the diaphragm, and a second sound inlet opening (1b, 2b, 3b), which leads to the back of the diaphragm, and in which the directional characteristic of each pressure gradient transducer (1, 2, 3) comprises an omni portion and a figure-of-eight portion and has a direction of maximum sensitivity, i.e. the main direction, and in which the main directions (1c, 2c, 3c) of the pressure gradient transducers (1, 2, 3) are inclined relative to each other, characterized in that the acoustic centers (101, 201, 301) of the three pressure gradient transducers (1, 2, 3) lie within an imaginary sphere (O) whose radius (R) corresponds to the double of the largest dimension (D) of the diaphragm (100, 200, 300) of one of said pressure gradient transducer (1, 2, 3), and that the projections of the main directions (1c, 2c, 3c) of the three pressure gradient transducers into a base plane spanned by the first sound inlet openings (1a, 2a, 3a) of the three pressure gradient transducers (1, 2, 3) enclose an angle with each other, whose values lie between 110° and 130°.
Microphone arrangement according to Claim 1, characterized by the fact that the acoustic centers (101, 201, 301) of the pressure gradient transducers (1, 2, 3) lie within an imaginary sphere (O) whose radius (R) corresponds to the largest dimension (D) of the diaphragm (100, 200, 300) of one of said transducer (1, 2, 3).
Microphone arrangement according to one of Claims 1 to 2, characterized by the fact that the projections of the main directions (1c, 2c, 3c) of the three pressure gradient transducers (1, 2, 3) enclose an angle of essentially 120° with each other in a plane, in which the first sound inlet openings (1 a, 2a, 3a) also lie.
Microphone arrangement according to one of Claims 1 to 3, characterized by the fact that the pressure gradient transducers (1, 2, 3) are arranged within a boundary (20).
Microphone arrangement according to one of Claims 1 to 4, characterized by the fact that in each of the three pressure gradient transducers (1, 2, 3), the first sound inlet opening (1a, 2a, 3a) and the second sound inlet opening (1b, 2b, 3b) are arranged on the same side, the front of a housing.
Microphone arrangement according to one of Claims 3 to 5, characterized by the fact that the front sides of the three pressure gradient transducers (1, 2, 3) are arranged flush with the boundary.
Microphone arrangement according to one of Claims 1 to 4, characterized by the fact that in each of the pressure gradient transducers (1, 2, 3), the first sound inlet opening (1a, 2a, 3a) is arranged on the front of the capsule housing and the second sound inlet opening (1b, 2b, 3b) is arranged on the back of the capsule housing.
Microphone arrangement according to one of Claims 1 to 7, characterized by the fact that the three pressure gradient transducers (1, 2, 3) lie against one of the three lateral surfaces of an imaginary equilateral triangular prism.
Microphone arrangement according to one of Claims 1 to 7, characterized by the fact that the three pressure gradient transducers (1, 2, 3) lie against one of the three lateral surfaces of an imaginary regular pyramid with an equilateral triangular base.
Microphone arrangement according to one of Claims 1 to 9, characterized by the fact that the three pressure gradient transducers (1, 2, 3) are arranged in a common capsule housing (21).
Microphone arrangement according to one of Claims 1 to 10, characterized by the fact that an additional pressure gradient transducer (4) is provided having a diaphragm, with the additional pressure gradient capsule (4) having a first sound inlet opening (4a), which leads to the front of the diaphragm, and a second sound inlet opening (4b), which leads to the back of the diaphragm, and that the acoustic center of the additional pressure gradient transducer (4) lies within the imaginary sphere (O) in which the acoustic centers of the other pressure gradient transducers (1, 2, 3) also lie, and that the main direction (4c) of the additional pressure gradient transducer (4) lies essentially normal to the base plane.
Microphone arrangement according to Claim 11, characterized by the fact that the additional pressure gradient transducer (4) has a figure-of-eight characteristic.
Method for synthesizing one or more microphone signals from a microphone arrangement according to one of Claims 1 to 11, characterized by the fact that, starting from the signals (K1, K2, K3) of the three pressure gradient transducers (1, 2, 3), a B-format (W, X, Y) is formed, which comprises an omni signal (W) and two figure-of-eight signals (X, Y) which are orthogonal to each other.
Method according to Claim 13, characterized by the fact that, starting from the signals (K1, K2, K3) of the three pressure gradient transducers (1, 2, 3) and the signal (K4) of the additional pressure gradient transducer (4), a B-format (W, X, Y, Z) is formed, which contains an omni signal (W) and three figure-of-eight signals (X, Y, Z) which are orthogonal to each other.
Method according to one of Claims 13 to 14, characterized by the fact that normalization of the B-format is carried out, according to which the B-format signals (W, X, Y) assume the following form: $W = \frac{(K 1 + K 2 + K 3)}{3} * \frac{(a + b)}{a},$
$X = \frac{(K 2 - K 3)}{\sqrt{3}} * \frac{(a + b)}{b}, Y = \frac{(2 K 1 - K 2 - K 3)}{3} * \frac{(a + b)}{b},$

in which a represents the weighting factor for the omni portion and b the weighting factor for the figure-of-eight portion of signals (K1, K2, K3) of the three pressure gradient capsules (1, 2, 3), in which signals (K1, K2, K3) can be described by the expression: $Kx = \frac{1}{a + b} (a + b \cos (ϕ)) .$
Method according to one of Claims 13 to 15, characterized by the fact that two signals (s1, s2) synthesized from the B-format (W, X, Y) are formed, the first signal (s1) containing an omni portion (W) and at least one figure-of-eight portion (X, Y), and the second signal (s2) containing at least one figure-of-eight portion (X, Y), that the signals (s1, s2) are transformed into the frequency range (S1(ω), S2(ω)) and are subtracted from each other, independently of their phases, by spectral subtraction, and that the signal then forming with the phase (Θ1(ω)) of the signal (S1(ω)), originating from the first signal (s1), which also contains an omni portion (W), is provided before it is back-transformed into the time range.
Method according to Claim 16, characterized by the fact that the frequency responses of the B-format signals (W, X, Y, Z) are equalized to each other before formation of the synthesized signals (s1, s2).